1. Field of the Invention
This invention relates to optical transmissions over a fiber optic cable. Specifically, the present invention relates to a system and method of coherent detection of optical signals by utilizing digital signal processing to recover signals.
2. Description of the Related Art
The use of optical fiber cables for the transmission of information was introduced several years ago. Recently, with the hunger by users for the rapid transmission of large amounts of information, the utility of the transmission of optical signals is particularly evident. [see “optical Communication Systems” by J. Gowar (Gowar) and “Fiber-optic Communication Systems” by G. Agrawal (Agrawal 2)]. The transmission of this information typically takes the form of binary digital signs (i.e., logical “1”s and “0”s. In addition, fiber optics is utilized to transport analog signs, such as cable television signals.
In the 1990s, optical amplifiers were deployed in telephonic and cable television networks. Typically, erbium doped fiber amplifiers (EDFAs) were employed. The amplifiers amplify the optical signals and overcome the loss of a signal transmitted over the fiber without the need to detect and retransmit the signals. In addition, the 1990s saw the introduction of wavelength division multiplexing (WDM) on a commercial level, which increased the information carrying capacity of the fiber by transmitting several different wavelengths in parallel. During WDM, different wavelengths originate and terminate at the same place, but in some cases, wavelengths are added or dropped in route to a destination.
With optical signal transmission systems, each system has a transmitter to emit light modulated with information through the fiber optics cable and a receiver, which detects the light and recovers the information. The transmission unit contains a light source, usually a single longitudinal mode semiconductor laser. Information is imposed on the light by direct modulation of the laser current, or by external modulation (by applying a voltage to a modulator component that follows the laser signal). The receiver utilizes a photodetector, which converts light into an electric current.
There are currently two ways to detect the light, direct detection and coherent detection. Existing transmission systems all use direct detection techniques for detecting the light. Although coherent detection techniques are more complex, they do offer some distinct advantages. One of the primary advantages is that it effectively provides signal gain. However, the EDFA offered the same advantage, and it was found to a more cost-effective solution.
Current transmission systems impose information on the amplitude (or intensity/power) of the signal. The light is switched on to transmit a “1” and off to transmit a “0.” In the case of direct detection techniques, the photodetector is presented with the on-off modulated light, and consequently the current flowing through it is a replica of the optical power. After amplification, the electrical signal is passed to a decision circuit, which compares it to a reference value. The decision circuit then outputs an unambiguous “1” or “0.”
There are many kinds of on-off modulation formats. The simplest one is unchirped non-return-to-zero (unchirped NRZ), where the optical power and the phase of the optical wave are kept constant during the transition between a pair of consecutive “1” symbols. Other transmission formats are chirped NRZ, return-to-zero (RZ) [see “Comparison between NRZ and RZ signal formats for in-line amplifier transmission in the zero-dispersion regime” by Matsuda (Matsuda)], carrier suppressed RZ [see “100 GHZ-spaced 8×43 Gbit/s DWDM unrepeatered transmission over 163 km using duobinary-carrier-suppressed return-to-zero format” by Y. Miyamoto et al. (Miyamoto)] and phase shaped binary transmission (PSBT) [see “The phase-shaped binary transmission (PSBT): a new technique to transmit far beyond the chromatic dispersion limit” by D. Penninckx et al. (Penninckx)].
There is also another class of modulation formats where information is encoded on the phase of the optical signal, such as optical differential phase shift keying (oDPSK). A photodetector does not respond to changes in the phase of the light falling on it, so a passive component called a discriminator is used before the photodetector. The discriminator converts the changes in phase into changes in power, which the photodetector may detect.
As discussed above, the photodetector does not respond to the phase of an optical wave. If two wavelengths are input to the photodetector, for example, the photodetector does not distinguish between the two wavelengths. The WDM systems utilize passive optical filter components to separate out the different wavelength channels at the receiver terminal, so each photodetector detects only one channel. This approach places a limit on how close the channels may be spaced, primarily based upon the optical filter's ability to pass one channel and reject its neighbors.
The coherent detection techniques treat the optical wave in a manner similar to radio wave reception by inherently selecting one wavelength and responding to its amplitude and phase. FIG. 1A illustrates a simplified block diagram of a single ended basic coherent receiver 16 in an existing fiber optics system. FIG. 1B illustrates a simplified block diagram of a coherent receiver 21 using balanced detection with two photodetectors 27 and 29 in an existing fiber optics system. An incoming signal 18 is combined with light from a local oscillator (LO) 20, which has close to the same state of polarization (SOP) and the exact or very similar wavelength. When the combined signals are detected, the photocurrent contains a component at a frequency which is the difference between the signal and the local oscillator optical frequencies. This difference frequency component contains all the information (amplitude and phase) that is on the optical signal. Because the new carrier frequency is much lower, typically a few GHz instead of 200 THz, all information on the signal can be recovered using standard radio demodulation methods. Coherent receivers see only signals close in wavelength to the local oscillator. Therefore, tuning the LO wavelength provides the functionality of a built-in tunable filter.
The coherent detection process may be explained with several mathematic equations. The following description utilizes complex notation for sinusoids that are summarized in Appendix A. The electric field of the signal may be written as:
  Re  [                    E        s            ⁡              (        t        )              ⁢          ⅇ                        ⅈω          s                ⁢        t              ]where Es(t) is the slowly varying envelope containing the information encoded on amplitude and phase of the optical signal. Similarly, the electric field of the local oscillator may be described as:
  Re  [            E      LO        ⁢          ⅇ                        ⅈω          LO                ⁢        t              ]where ELO is a constant for a local oscillator. The electric field of the light arriving at the photodetector 29 in the top branch of FIG. 1B (or the photodetector 24 in FIG. 1A) is the sum of the two electric fields:
      E    1    =      Re    ⁡          [                                                  E              s                        ⁡                          (              t              )                                ⁢                      ⅇ                          ⅈ              ⁢                                                          ⁢                              ω                s                            ⁢              t                                      +                              E            LO                    ⁢                      ⅇ                          ⅈ              ⁢                                                          ⁢                              ω                LO                            ⁢              t                                          ]      and the optical power is:
                                                                        P                1                            =                                                                    E                    1                    *                                    ⁢                                      E                    1                                                  =                                                      (                                                                                                                        E                            s                            *                                                    ⁡                                                      (                            t                            )                                                                          ⁢                                                  ⅇ                                                                                    -                                                              ⅈω                                s                                                                                      ⁢                            t                                                                                              +                                                                        E                          LO                          *                                                ⁢                                                  ⅇ                                                                                    -                                                              ⅈω                                LO                                                                                      ⁢                            t                                                                                                                )                                    ⁢                                      (                                                                                                                        E                            s                                                    ⁡                                                      (                            t                            )                                                                          ⁢                                                  ⅇ                                                                                    ⅈω                              s                                                        ⁢                            t                                                                                              +                                                                        E                          LO                                                ⁢                                                  ⅇ                                                                                    ⅈω                              LO                                                        ⁢                            t                                                                                                                )                                                                                                                                          P                1                            =                                                                                                                                      E                        s                                            ⁡                                              (                        t                        )                                                                                                  2                                +                                                                                                E                      LO                                                                            2                                +                                  2                  ⁢                                      Re                    [                                                                                            E                          s                                                ⁡                                                  (                          t                          )                                                                    ⁢                                              E                        LO                        *                                            ⁢                                              ⅇ                                                                              ⅈ                            ⁡                                                          (                                                                                                ω                                  s                                                                -                                                                  ω                                  LO                                                                                            )                                                                                ⁢                          t                                                                                      ]                                                                                                          (        1        )            
In the case of single ended detection, only one output of the combiner is used. |ELO|2 is constant with time. |Es(t)|2 is relatively small, given that the local oscillator power is much larger than the signal power. In addition, for the phase shift keying (PSK) and frequency shift keying (FSK) modulation formats |Es(t)|2 is constant with time. The dominant term in equation 1 is the beat term
      Re    [                            E          s                ⁡                  (          t          )                    ⁢              E        LO        *            ⁢              ⅇ                              ⅈ            ⁡                          (                                                ω                  s                                -                                  ω                  LO                                            )                                ⁢          t                      ]    .
The output of the lower branch is the difference of the two electric fields, and the optical power is:
                              P          2                =                                                                                            E                  s                                ⁡                                  (                  t                  )                                                                    2                    +                                                                  E                LO                                                    2                    -                      2            ⁢                          Re              [                                                                    E                    s                                    ⁡                                      (                    t                    )                                                  ⁢                                  E                  LO                  *                                ⁢                                  ⅇ                                                            ⅈ                      ⁡                                              (                                                                              ω                            s                                                    -                                                      ω                            LO                                                                          )                                                              ⁢                    t                                                              ]                                                          (        2        )            The other mode of detection is balanced detection, where the electrical circuitry after the photodetectors evaluates the difference in photocurrent between the two detectors:
                                          P            1                    -                      P            2                          =                  4          ⁢                                          ⁢                      Re            ⁡                          [                                                                    E                    s                                    ⁡                                      (                    t                    )                                                  ⁢                                  E                  LO                  *                                ⁢                                  ⅇ                                                            ⅈ                      ⁡                                              (                                                                              ω                            s                                                    -                                                      ω                            LO                                                                          )                                                              ⁢                    t                                                              ]                                                          (        3        )            Balanced detection produces the beat term directly. With balanced detection, there is no need for the constraint that the local oscillator power should be greater than the signal power. It has the additional advantage that noise on the local oscillator is subtracted out.
The following equations refer to the beat term directly, and it is assumed that this term is obtained by single ended detection (without the contribution of other terms) or by balanced detection.
There are two modes of coherent detection: homodyne and heterodyne. With homodyne detection, the frequency difference between the signal and the local oscillator is zero. The local oscillator laser has to be phase locked to the incoming signal in order to achieve this. For homodyne detection the term ei(ωs−ωLO)t is 1, and the beat term becomesRe[Es(t)ELO*]
For the binary phase shift keying (BPSK) modulation format, Es(t) takes on the value 1 or −1 depending on whether a logical “1” or “0” was transmitted, and the decision circuit can simply act on the beat term directly. Homodyne detection requires that the bandwidth of the photodetector and the subsequent components be close to the bit rate. In addition, homodyne detection gives a better sensitivity than any other way of detecting the signal. Also homodyne detection has an inherent ultranarrow optical filtering capability, in that all regions of the optical spectrum, which are more that the detector bandwidth away from the local oscillator, are rejected. This feature means that homodyne detection can support a higher density of WDM channels than by using passive optical filters for WDM demultiplexing. The homodyne detection method has the disadvantage that the local oscillator must be phase locked to the signal. The local oscillator and signal lasers must be narrow linewidth lasers, such as external cavity semiconductor lasers, which are typically more expensive than the distributed feedback (DFB) laser. Additionally, some polarization management methods do not work with homodyne detection.
With heterodyne detection, there is a finite difference in optical frequency between the signal and local oscillator. All the amplitude and phase information on the signal appears on a carrier at angular frequency (ωs−ωLO), known as the intermediate frequency (IF), which can be detected using standard radio detection methods (e.g., synchronous detection, envelope detection or differential detection). Heterodyne detection has the advantage that the local oscillator does not need to be phase locked, and a DFB laser can be used for the LO and the signal lasers. Also it is possible to employ signal processing in the IF to compensate for chromatic dispersion, which is considered impossible to do with homodyne detection by existing techniques. The heterodyne detection processes suffer from the disadvantage that the difference frequency must be at least equal to half the optical spectral width of the signal, about 0.75 times the symbol rate, to avoid a penalty from self-imaging, which requires the bandwidth of the photodetector to be at least 1.5 times the symbol rate. The sensitivity of heterodyne detection is 3 dB worse than homodyne detection. In addition, for heterodyning to work, there must be an empty region in the optical spectrum adjacent to the signal being detected, which constrains the density at which WDM channels can be packed.
A system and method is needed which includes all the advantages of homodyne detection in conjunction with all of the advantages of heterodyne detection. In addition, a system and method is needed which can vary the chromatic dispersion compensation and subtract cross talk from other WDM channels.
Thus, it would be a distinct advantage to have a system and method which incorporates coherent detection of optical signals with digital signal processing to recover a signal when a local oscillator is not phase locked to the signal. It is an object of the present invention to provide such a system and method.